1. Field of the Invention
The present invention relates to an air flow measuring device for measuring a flow amount of air flowing in a duct.
2. Description of the Related Art
An air flow measuring device described in U.S. Pat. No. 5,804,718 (corresponding to JP 9-287991A) is provided for measuring a flow amount of intake air flowing into an internal combustion engine. As shown in FIG. 7, the air flow measuring device includes a sensor body 110 disposed in an intake air passage 100 of the internal combustion engine. The sensor body 110 is provided with a bypass passage 120 into which a part of air flowing in the intake air passage 100 is introduced, and a communication hole 130 through which an inlet side of the bypass passage 120 and an outlet side of the bypass passage 120 communicate with each other. A flow amount sensor 140 is located in the bypass passage 120 to measure the flow amount of air flowing into the interior of the bypass passage 120.
However, in the flow amount measuring device, the flow amount sensor 140 may be difficult to accurately detect the flow amount of air due to dispersion, disturbance or/and variation of an air flow from an air cleaner that is located upstream of the intake passage 100. That is, when a bias flow shown in FIG. 7 is caused at an upstream side of the sensor body 110 due to an air disturbance or the like, a separation of an airflow is caused around the sensor body 110 and near the outlet side of the bypass passage 120. In this case, a large swirl of the air flow may be caused around the sensor body 110 and near the outlet side of the bypass passage 120, as shown in FIG. 7. Because of the large swirl of the air flow, a pressure difference between the inlet side and the outlet side of the bypass passage 120 is greatly varied, and thereby a flow speed of air flowing in the bypass passage 120 becomes unstable. As a result, a variation in output of the flow amount sensor 140 becomes larger, and thereby detection accuracy of the flow amount sensor 140 is deteriorated.